Public Health Emergency Preparedness

Chapter 11: Yersinia Pestis (Plague)

This chapter describes the assumptions in the pneumonic plague scenario, including:

• The severity categories.
• The arrival pattern of casualties at the hospital(s).
• The length of stay by hospital unit (i.e., ED, ICU, and the floor).
• The path probability within the hospital(s) and the length of stay.
• The overall outcome probabilities (i.e., probability of discharge and probability of death).
• The assumed level of resource consumption per patient per day per hospital unit.

Footnotes in the text of a particular section refer to references at the end of the section. In the absence of specific references, parameter estimates were obtained from general references listed in the Hospital Module section.

11.1 Severity Categories

For the plague scenario, patients arrive at the hospital(s) in one of two conditions:

1. Moderate: Patient requires hospitalization, but not in the ICU.
2. Severe: Patient requires hospitalization in the ICU.

If the user specifies only the number of casualties, the model assumes the casualties arriving at the hospital(s) are distributed as follows:

Casualty Condition	Percent
Moderate: Patient requires hospitalization, but not in the ICU	100%
Severe: Patient requires hospitalization in the ICU	0%

That is, if a number of moderate and severe casualties is not specified, the model assumes that all casualties arrive in a moderate condition (i.e., individuals do not wait until they have severe symptoms to present at the hospital).

Very little has been written detailing the hospital-based treatment of plague, and virtually all of these cover bubonic plague, not the pneumonic plague of the hypothetical scenario. Of these, only one article is a true primary case review pertaining to hospital logistics for plague patients.¹ This article provides low, mode, and high average lengths of stay for patients treated with one of three antibiotics (streptomycin, gentamicin, or tetracycline) of 3.8, 6.5, and 9.2 days, respectively. A second article reports an approximate 15 percent case fatality ratio in India,²⁸ a mortality burden that is noted in Harrison's Principles of Internal Medicine, as well as the Working Group for Civilian Biodefense's review of non-pneumonic plague.²⁵

Since the scenario under consideration is pneumonic plague, the mortality rate was doubled to provide the baseline value of 34 percent (assuming mode length of stay and low mortality). Exhibit 10 shows that mortality from severe plague varies from 27 percent to almost 70 percent depending on length of stay and mortality rate assumptions. Mortality from moderate disease remains very low due to how these cases were defined in the casualty generation module.

Exhibit 10: Mortality Estimates for Plague Scenario

11.2 Casualty Arrival Pattern

Casualties are assumed to present at the hospital(s) as symptoms appear over a 10-day period. The pattern increases over the first half of this period as new individuals are infected with Yersinia pestis. During the second half of this period, the arrival rate decreases sharply, because of the effect of a prophylaxis campaign that is assumed to be in place.

11.3 Length of Stay by Hospital Unit

The assumed average lengths of stay (in days) of patients in the ED, in the ICU, and on the floor are:

Average Length of Stay by Hospital Unit	Moderate	Severe
ED	1	1
Floor, not via ICU	5.1	5.1
Floor, via ICU	3.9	3.9
ICU	3.9	3.9

11.4 Combined Path Probabilities and Lengths of Stay

The table below shows the assumed probabilities of different "paths" through the hospital(s).

Path	Moderate	Severe
ED → Discharge	0%	0%
ED → Death	0%	0%
ED → Floor → Discharge	98%	0%
ED → Floor → Death	0%	0%
ED → Floor → ICU → Death	0%	0%
ED → Floor → ICU → Floor → Discharge	0%	0%
ED → Floor → ICU → Floor → Death	0%	0%
ED → ICU → Death	1%	28%
ED → ICU → Floor → Discharge	1%	66%
ED → ICU → Floor → Death	0%	6%

The breakdown of length of stay by patient type summed over all paths is:

Average Length of Stay by Patient Outcome	Moderate	Severe
Survivors	6.04	9.00
Fatalities	3.90	4.06
Average Combined	6.02	7.33

11.5 Overall Outcome Probabilities

Based on these inputs, the overall discharge and death probabilities are:

Outcome	Moderate	Severe
Discharge	99%	66%
Death	1%	34%

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